1. Field of the Invention
This invention relates to a data transmission system, and more particularly to an improvement of transmitting picture data after compression thereof through a communication line or the like.
2. Description of the Prior Art
Conventionally, in a data transmission system for transmitting picture data and audio data to remote places or recording these data in a digital system recording media, the picture data and audio data are compressed by utilizing correlation between data thereof in order to obtain efficient transmission of picture data having large data quantities.
Assuming an imaginary transmission line free from noise, the compression and transmission factors of data transmitted from the data transmission system 1 can be thought to be as follows (FIG. 1).
When coding data u.sub.L output from a data source through an encoder 2 into a digital signal and transmitting data c.sub.k obtained by data compression through a noise-free transmission line 3, and denoting decoded data from a decoder 4 by v.sub.L, the compression factor C of the transmitted data can be expressed by following formula: ##EQU1##
Denoted by A is the number of bits per unit time of data u.sub.L before compression, and by A.sub.R the number of bits per unit time of data v.sub.L after compression. The data transmission system 1 is adapted to transmit data with the compression factor C thereof which is controlled to be constant irrespective of the kind of the transmitted data.
However, depending on the kind of the transmitted picture contents or audio contents the control of the compression factor C to a constant value sometimes results in considerable deterioration of the picture quality or audio quality.
For example, with pictures having less inter-frame correlation, such as those having many scene changes, it is difficult to compress data. Therefore, there occurs great difference of the distortion measure d (u.sub.L, v.sub.L) even if the compression factor C is controlled to be constant.
A distortion measure d (u.sub.L, V.sub.L) is the double sum mean distortion which is defined by following equation: ##EQU2##
It may be thought to hold the distortion measure d (u.sub.L, v.sub.L) within a predetermined value. In this case, depending on transmitted data the compression factor C is infinitely small, and the transmission capacity of the transmission line 3 is exceeded. In such a case, the data transmission system 1 can no longer be used.
This is so in that a small compression factor may dictate a data quantity corresponding to a data transmission rate more than the maximum rate, at which the data transmission system 1 can transmit data in unit time, thus resulting in failure of transmission.